Charge amplifiers are used in a number of electronic devices in which very small charges must be detected and amplified, including photodetectors, CCD imaging devices, fiber optic receivers, accelerometers and virtually all Micro-Electro-Mechanical Systems (MEMS). An ideal charge amplifier 10 is shown in FIG. 1. Charge amplifier 10 includes a high-gain operational amplifier (“opamp”) A having a non-inverting input terminal 12, an inverting input terminal 14 and an output terminal 16. A feedback capacitor Cf is connected between the inverting input terminal 14 and the output terminal 16. The transfer function of charge amplifier 10 is:                                           V            out                                Q            in                          =                                            -              1                                      C              f                                .                                    Eq        .                  (          1          )                    Also,                                           V            out                                I            in                          =                                            -              1                                      sC              f                                .                                    Eq        .                  (          2          )                    
In an ideal environment, the charge amplifier 10 is used to measure a charge Qin at input terminal 14 which is alternating in polarity, such that the average value of the output Vout at output terminal 16 is 0V. Such an ideal output value would prevent opamp A from saturating. However, in a typical operating environment, the opamp 10 has an input leakage current which acts as a constant Iin and causes the output terminal Vout of the charge amplifier 10 to drift toward the supply rail and, as a result, to saturate.
In order to prevent the saturation of the charge amplifier by minimizing or eliminating the input leakage current, a DC conductive parallel path must be present across the capacitor Cf. A simple prior art method for providing this path is to connect a high value resistor Rf in parallel with capacitor Cf, as shown in the charge amplifier 20 of FIG. 2. The transfer function of the charge amplifier 20 is:                                           V            out                                I            in                          =                                            -                              R                f                                                                                      sC                  f                                ⁢                                  R                  f                                            +              1                                .                                    Eq        .                  (          3          )                    
When Iin is of a sufficiently high frequency such that sCfRf>>1,                                           V            out                                I            in                          =                              -            1                                sC            f                                              Eq        .                  (          4          )                    which approximates the transfer function (Eq. (2)) of charge amplifier 10 of FIG. 1. However, in order for this approximation to be valid at lower input frequencies, i.e. 10-100 kHz, the value of the resistance Rf must be in the range of 10-1000 MΩ. For example, for an input frequency of 20 kHz and Cf=2pF, the resistance Rf must have a value of 200 MΩ to enable the charge amplifier 20 to approximate the ideal transfer function for charge amplifier 10 shown in Eq. 2. Such high resistance values are not compatible with current CMOS integrated circuit manufacturing processes. Therefore, in order to construct such a charge amplifier, a large, external, discrete resistor must be used, resulting in greater cost, volume and weight of the charge amplifier, as well as increased output noise of the charge amplifier due to the added stray capacitance and thermal noise associated with the external resistor.
Another prior art method of minimizing the input leakage current involves connecting a MOS transistor which is configured as a switch in parallel with capacitor Cf. The MOS switch is periodically conducted at a low duty cycle rate, which enables the charge amplifier to obtain the transfer function of Eq. 2 for the periods of time that the MOS switch is open. However, when the switch is closed, the output of the charge amplifier goes to zero. While this system is fully integratable, it does not allow continuous time signal processing, and thus portions of the Qin signal are lost.
Another prior art method of minimizing the input leakage current involves connecting a MOS transistor which is configured as a diode, with its gate connected to its drain, in parallel with capacitor Cf. This diode operates to conduct the leakage current and presents a very large resistance to the charge amplifier. However, such a diode configuration causes large (on the order of 1V) and varying DC voltages to be present at the output of the charge amplifier. Furthermore, the extremely large transistor resistance may result in excessive sensitivity to a very low frequency charge, resulting in an acceleration-sensitive charge amplifier.
Yet another method of minimizing the input leakage current involves connecting an operational trans-impedance amplifier (“OTA”) in parallel with the capacitor Cf, as shown in charge amplifier 30 in FIG. 3. Charge amplifier 30 includes OTA B having a non-inverting input terminal 32 connected to the output terminal 16 of opamp A and an inverting input terminal 34 connected to ground. The output terminal 36 is connected to the inverting input terminal 14 of opamp A. The OTA B is a MOSFET device having a transfer function:Iout=gm[(Vin−)]  Eq. (5). In operation, the OTA B outputs the leakage current necessary to servo the output of opamp A to zero volts. In the case of charge amplifier 30, the transconductance gm must be sufficiently small such that the reactance of capacitor Cf will dominate at the input signal frequency. This device suffers from the addition of noise to the output due to the active components of the OTA B. Furthermore, as is typical for all OTA's, when the required low transconductances are synthesized, a relatively large input offset voltage results due to transistor mismatch and leakage.
Accordingly, the prior art attempts to stabilize DC voltage and minimize the input leakage current in a charge amplifier are not fully integratable and therefore suffer from increased size, cost and volume, increase the noise and offset voltage in the amplifier and/or cannot process input signals in continuous time.